This invention relates generally to the art of producing self-tapping threaded fasteners and more particularly to the art of producing such fasteners from a stainless steel material.
A type of stainless steel material, namely 300 series, has for many years been the primary material utilized for producing highly corrosive-resistant devices, such as threaded fasteners. However, such a material, which is typically referred to as 18-8 stainless steel, referring to the percentages of chromium and nickel-like components, are austenitic and nonheat-treatable. Thus, these materials have been confined to usages where high hardness levels are not required. In the environment of self-tapping screws, it is apparent that such hardness levels are required and typically a range of hardness of 45-50 R.sub.C is necessary in order to tap or form threads in a carbon steel workpiece.
There have been numerous attempts to provide a stainless steel material with the hardness necessary to perform adequately in a tapping environment. Typical of such attempts are the use of a 400 series stainless which is, at most, 12% chromium. Such material is heat-treated and quenched to relieve stresses and then reheated to a moderate temperature. This produces a fastener which is hardened throughout in hardness ranges sufficient to tap but with a tendency to become brittle. However, since the chromium content is limited to 12%, such materials are not as corrosive-resistant as the 300 series, 18-8 material.
Other attempts to provide a heat-treatable characteristic to a stainless material with higher chromium content involve the use of precipitation hardening agents, such as titanium or columbium in the chemistry of the steel with subsequent age hardening steps. These techniques, however, tend to deplete the effective chromium and are, at most, a compromise solution.
Stainless steels which include 18% chromium and 18% of a nickel-type material are available and have been found to be hard enough to function in many tapping environments. However, this material is difficult to cold-head and thread roll because of its inherent hardness causing very short tool life in both such operations.
Other attempts to provide a complex treatment for the steel by heating or the addition of components, such as aluminum and critical quantities of chromium, nickel and carbon have been attempted. All of which appear to be expensive and difficult to utilize in a high production fastener manufacturing situations again appear to provide only a compromise solution.